Protective enclosure for gas sensors

ABSTRACT

A small-sized, portable enclosure protects a gas sensor against degradation due to environmental exposure and changes in atmospheric conditions. The protective enclosure includes an inlet for introduction of a gas into the enclosure, an outlet for release of the gas upon completion of a sensing run, and a number of in-line filters that remove from the inflowing gas sample analytes, contaminants, and other materials that can compromise the integrity of the sensor or cause the sensor to degrade over time. The enclosure does not include any filters during the measurement phase of the sensing run in order to allow the gas sensor to accurately measure an unmodified gas mixture and/or analyte.

TECHNICAL FIELD

The present invention relates generally to gas sensors and morespecifically to a protective enclosure for gas sensors.

BACKGROUND OF THE INVENTION

Electronic nose devices are attracting increasing attention as theInternet of Things (IoT) is taking shape. Electronic nose devicesgenerally include a multi-gas sensor array whereupon exposure to agaseous odor, the collective output of the sensors associates a uniquefingerprint to the odor, which is used to discriminate the individualvolatiles of the gaseous odor. The individual volatiles are used tolabel the detected odor and/or identify process anomalies and/ordeviations from given standards. Gas sensors are important for IoTdevices since they provide the data that are used by cloud computing togenerate meaningful output through Machine Learning and artificialintelligence (AI).

Electronic nose devices currently in use suffer from gas sensordegradation. Aging and changes in atmospheric conditions lead gassensors to undergo changes in sensing material morphology, composition,electrical behavior over time, and surface poisoning from externalcontamination, etc. Sensor degradation ultimately leads to a decrease inperformance, compromising the proper functioning and reliability of anelectronic nose device. Attempts to alleviate gas sensor degradationthrough techniques, such as storing sensors under vacuum and onlyinjecting gaseous analytes when gas sensing measurements are taken haveproven to be ineffective for several reasons, including the requirementfor bulky expensive equipment and limitations on the timing at which themeasurements may be taken. The use of filters to remove deleterious andunwanted gases during the sensing measurements has also beenineffective, because filters are never completely specific and canfilter out a broad range of gases, including analytes of interest.

There remains a need in the art for a gas sensing device that isspecific, efficient, and portable.

SUMMARY OF THE INVENTION

The present invention overcomes the need in the art by providing aprotective enclosure for gas sensors.

In one aspect, the present invention relates to a method, comprising:(i) providing an enclosure comprising an inlet, at least one outlet, atleast one filter, and a sensor that detects an analyte of interest; (ii)introducing a gas comprising the analyte of interest through the inlet,wherein the sensor detects the analyte of interest in the gas, the gasexits the enclosure through the at least one outlet after detection, thegas does not contact the at least one filter between input of the gasthrough the inlet and exit of the gas through the at least one outlet,and the gas further includes at least one deleterious compound thatdegrades performance of the sensor; (iii) introducing an additionalinflux of the gas through the inlet, wherein the additional influx ofthe gas passes through the at least one filter prior to contacting thesensor, wherein the at least one filter removes the at least onedeleterious compound from the additional influx of the gas; and (iv)allowing the additional influx of the gas that is free of thedeleterious compounds to remain in contact with the sensor.

In another aspect, the enclosure further comprises at least two valves,one of which opens and closes the inlet and another of which opens andcloses at least one outlet.

In another aspect, the inlet and one of the at least one outlet are openat step (ii) and both the inlet and the at least one outlet are closedat step (iv).

In a further aspect, the at least one filter is a retractable filtersituated at the inlet, wherein the at least one retractable filterretracts at step (ii) so that the gas may enter the enclosure as anunfiltered gas, and is repositioned at the inlet in step (iii) so thatthe additional influx of gas must pass through the at least oneretractable filter to contact the gas sensor.

In another aspect, steps (ii) and (iii) operate in a time-frame rangingfrom one second to no longer than 12 hours.

In another aspect, the enclosure further comprises a sensing gas line, apurge gas line, and a valve at the inlet to direct the gas from theinlet into the sensing gas line and/or the purge gas line, wherein thesensing gas line has no filters and the at least one filter is situatedin the purging gas line.

In one embodiment, the present invention relates to a device comprising:an enclosure; an inlet for input of a gas; a sensing gas line with nofilters; a purge gas line with at least one filter to remove deleteriouscompounds from the gas; an inlet valve to open and close the inlet anddirect flow of the gas into the sensing gas line and/or the purge gasline; a gas sensor with access to the sensing gas line and the purge gasline, wherein the gas sensor has (i) a first valve to cut off access ofthe gas sensor to the sensing gas line and (ii) a second valve to cutoff access of the gas sensor to the purge gas line; at least one outletto exhaust the gas from the sensing gas line and the purge gas line; andat least one outlet valve to direct the exhaust from the sensing gasline and purge gas line out of the enclosure, wherein the at least oneoutlet valve is a one-way valve that prevents outside air from enteringthe enclosure.

In a further embodiment, the gas is an analyte of interest that entersthe enclosure through the inlet, the inlet valve directs the analyte ofinterest into the sensing gas line for measurement by the gas sensor,and after measurement, the analyte of interest leaves the enclosurethrough one of the at least one outlet.

In another embodiment, the gas is a flushing gas that enters theenclosure through the inlet, the inlet valve directs the flushing gasinto the purge gas line, wherein the flushing gas passes through the atleast one filter, over the gas sensor, and exits the enclosure throughone of the at least one outlet.

In a further embodiment, the inlet valve closes the inlet, and the atleast one outlet valve closes the at least one outlet, thereby sealingthe enclosure so that the flushing gas circulates within the sealedenclosure.

In another embodiment, the present invention relates to a devicecomprising: an enclosure; an inlet for input of a gas; an inlet valvefor closing the inlet; at least one retractable filter for removingdeleterious compounds from the gas, wherein the at least one retractablefilter is situated at the inlet; an outlet for exit of the gas; anoutlet valve for closing the outlet; and a gas sensor with access to theinlet and the outlet, wherein the gas sensor is (i) sealed from theinlet via closure of the inlet valve and (ii) sealed from the outlet viaclosure of the outlet valve.

In a further embodiment, the gas is an analyte of interest that entersthe enclosure through the inlet where the at least one filter removesdeleterious compounds from the gas, wherein the filtered gas passes ontothe gas sensor for measurement and after measurement, the analyte ofinterest leaves the enclosure through the outlet.

In another embodiment, the gas is a flushing gas that enters theenclosure through the inlet, passes over the gas sensor, and remainswithin the enclosure until released by the outlet, wherein upon entry ofthe flushing gas into the gas sensor, the inlet valve and the outletvalve close and seal the enclosure, thereby preventing escape of theflushing gas.

In other aspects and embodiments, the sensor is selected from the groupconsisting of a metal oxide thin film, a metal oxide nanostructure, anda metal nanoparticle.

In further aspects and embodiments, the sensor is a metal nanoparticlesensor, wherein the metal is selected from the group consisting of gold,silver, platinum, and palladium.

In other aspects and embodiments, the gas sensor is a metal nanoparticleligated with a thiol.

In further aspects and embodiments, the metal nanoparticle sensor isligated with a thiol selected from the group consisting of ethanethiol,hexanethiol, octanethiol, decanethiol, dodecanethiol, and combinationsthereof.

In other aspects and embodiments, the at least one filter is selectedfrom the group consisting of activated carbon, silica gel, andcombinations thereof.

In further aspects and embodiments, the at least one filter is areplaceable filter.

In other aspects and embodiments, the at least one deleterious compoundis selected from the group consisting of ozone, silicone, chlorine,particulates, sediment, volatile organic compounds, and combinationsthereof.

Additional aspects and embodiments of the invention will be provided,without limitation, in the detailed description of the invention that isset forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an embodiment of the sensor enclosure describedherein.

FIGS. 2A-2C are schematics showing different states of the sensorenclosure of FIG. 1 . FIG. 2A shows a measurement phase; FIG. 2B shows aflush mode; and FIG. 2C shows an idle phase of the sensor enclosure.

FIG. 3 is a schematic of another embodiment of the sensor enclosure.

FIGS. 4A and 4B are graphs showing SERS (surface enhanced Ramanspectroscopy) spectra results for decanethiol-ligated gold nanoparticles(Au-NPs) (FIG. 4A) and hexanethiol-ligated Au-NPs (FIG. 4B) under thefollowing conditions: pristine (i.e., a fresh sample directly afterdeposition), ambient air (exposure to open-air), and sealed (within a 25mL contained volume).

FIG. 5 is a graph showing XPS (x-ray photoelectron spectroscopy) spectraresults for a pristine Au film, as well as decanethiol-ligated Au-NPsunder pristine conditions and after 40-day aged ambient air conditions.

FIGS. 6A and 6B are SEM (scanning electron microscopy) micrographs of ahexanethiol-ligated Au-NP immediately after deposition (FIG. 6A) afteraging in open ambient air for 13 days (FIG. 6B).

FIG. 7 is a graph showing the relative change in current (Response) overtime of a sensor comprised of decanethiol-ligated Au-NPs aged for 14days in a sealed container and exposed between t=100 sec to t=500 sec to12678 ppm of octane in an N₂ stream.

FIG. 8 is a graph showing SERS spectra results for decanethiol-ligatedAu-NPs under the following conditions: pristine; after 24 hours ofstorage in freely exchanging ambient air; and after 24 hours of flushingwith zero-air (synthetic mixture of 80% and 20% oxygen).

FIGS. 9A and 9B show the effects of ozone contamination ondecanethiol-ligated Au-NPs. FIG. 9A is a schematic depicting a set-upfor the introduction of ozone contaminant into zero-air atmosphere andthe subsequent flow over to a sensor. FIG. 9B is a graph showing SERSspectra results of decanethiol-ligated Au-NPs under the followingconditions: 24 hours of flushing with zero-air; and 24 hours of flushingwith zero-air followed by one hour where zero-air was enriched withozone.

DETAILED DESCRIPTION OF THE INVENTION

Set forth below is a description of what are currently believed to bepreferred aspects and/or embodiments of the claimed invention. Anyalternates or modifications in function, purpose, or structure areintended to be covered by the appended claims. As used in thisspecification and the appended claims, the singular forms “a,” “an,” and“the” include plural referents unless the context clearly dictatesotherwise. The terms “comprise,” comprises,” and “comprising,” as usedin the specification and appended claims, specify the presence of theexpressly recited components, elements, features, and/or steps, but donot preclude the presence or addition of one or more other components,elements, features, and/or steps.

As used herein, the terms “gas sensor” and “sensor” are used to refer toa single gas sensor or to a gas sensor array. Plural versions of theterms are used to refer to more than one gas sensor or gas sensor array.Unless expressly stated herein, reference to a single gas sensor in thefollowing description will also include more than one gas sensor orsensor array.

The present invention provides a small-sized, portable enclosure thatprotects a gas sensor against degradation due to environmental exposureand changes in atmospheric conditions. The protective enclosure includesan inlet for introduction of a gas into the enclosure, an outlet forrelease of the gas upon completion of a sensing run, and a number ofin-line filters. The purpose of the filters is to remove from theinflowing gas sample any analytes, contaminants, and other materialsthat can compromise the integrity of the sensor or cause the sensor todegrade over time. The enclosure does not include any filters during themeasurement phase of the sensing run in order to allow the gas sensor toaccurately measure an unmodified gas mixture and/or analyte. The filtersthat remove deleterious gaseous species that lead to degradation of gassensors are only activated during the flushing and idle periods of thegas sensing process. The filter thus serves to flush the sensor arraywith gas in order to protect it. The type of filters used within theenclosure may differ based upon various factors, including: the gasesintroduced into the enclosure, the gases intended to be removed, and/orthe materials used in the sensor array. In one embodiment, the filtersare replaceable once expended. Once the deleterious gases have beenremoved from the gas influx flow, the inflowing gas sample may be usedto flush the sensing array of materials of compounds that may otherwisedegrade the sensor during idle periods. By preventing sensor degradationduring the idle periods of the gas sensing process, the lifetime of thegas sensor can be increased, thus leading to a wider range ofapplications for any particular gas sensor.

Examples of gas sensors that may be used with the protective enclosuresdescribed herein, include, without limitation, gas chromatography massspectrometers (GCMSs), Fourier-transform infrared spectrometers (FTIRs),electrochemical (EC) sensors, quartz crystal microbalances (QCMs), metaloxide thin films, metal oxide nanostructures, and metal nanoparticlesensors, and combinations thereof. Examples of metal oxides that may beused for metal oxide thin films or nanostructures include, withoutlimitation, aluminum oxide (Al₂O₃), ceric dioxide (CeO₂), cuprous oxide(Cu₂O), cupric oxide (CuO), copper peroxide (CuO₂), copper(III) oxide(Cu₂O₃), indium oxide (In₂O₃), ferric oxide (Fe₂O₃), iron(II) oxide FeO,iron(II,III) oxide (Fe₃O₄), manganese dioxide (MnO₂), tin(IV) oxide(SnO₂), titanium dioxide (TiO₂), tungsten trioxide (WO₃), zinc oxide(ZnO), and combinations thereof. Examples of metal nanoparticle sensorsinclude, without limitation, gold (Au), silver (Ag), platinum (Pt), andpalladium (Pd) nanoparticle sensors. Metal nanoparticles will typicallybe ligated with a thiol (R—SH) for protective purposes. Examples ofthiols that may be ligated to metal nanoparticles includes, withoutlimitation, organothiols, such as ethanethiol, hexanethiol, octanethiol,decanethiol, dodecanethiol, and combinations thereof.

Examples of filtering materials or devices that may be used in the gassensor enclosures described herein include, without limitation,activated carbon filters, silica gel, catalytic converters, andcombinations thereof. Activated carbon filters may remove some or all ofthe following deleterious compounds from a gas sample: ozone, silicone,chlorine, particulates, sediment, volatile organic compounds (VOCs), andcombinations thereof. Silica gel filters may be used to remove moisture.Catalytic converters may be used to decompose ozone into oxygen bycatalyzing a metal selected from the group consisting of iridium (Ir),osmium (Os), palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium(Ru), and combinations thereof.

In order to control the flow of gas into, out of, and through theenclosure, the enclosure is equipped with one-way and two-way valves, asappropriate. Gas input flow may be driven from the environment by asmall pump or directly by an external source (e.g., a person exhalinginto the device). Valves may be operated using a program, such as forexample, LABVIEW® (National Instruments Corp., Austin, Tex.). One typeof valve that may be used in the enclosure at the inlet is a Y-valvethat may be adjusted to allow the gas flow to be directed towards thegas sensor for measurement (where the enclosure is free of filters) andlater adjusted so that the gas flow may circulate through the enclosureand encounter the filters that will clear the gas flow of deleteriouscompounds that might otherwise degrade the sensors.

With reference to FIG. 1 , in one embodiment, the gas sensor enclosuremay include a valve to control direction of gas flow duringsensing/purging/idle modes 1 and at least one in-line contaminantfilter. Examples of in-line contaminant filters include, withoutlimitation, an in-line contaminant filter 2 to remove moisture (e.g.,silica gel) and an in-line contaminant filter 3 to remove/convert ozone(e.g., activated carbon). Additional in-line contaminant filters 4 maybe added to the enclosure. The in-line contaminant filters 2, 3, and 4can be removed and replaced as the filters become spent. The enclosuremay also include a sensor housing 5 and several one-way and two-wayvalves, such as for example, a one-way valve to prevent flow duringsensing and to allow flow during purging 6; a one-way exhaust valve toprevent contamination of sensors by unfiltered outside atmosphere,exhaust sensing gas flow, and direct purge gas flow 7; a two-way valveto allow forward flow of the sensing and purging gas and which seals toprotect the sensors 8; and a one-way valve to purge exhaust gas flow 9.The two-way valve 8 will seal in order to protect the sensors and theone-way valve 9 will be closed during the sensing mode.

FIGS. 2A-C illustrates three different modes of application of theexemplary gas sensor enclosure of FIG. 1 . The different modes mitigatedegradation of the gas sensors and/or gas sensor arrays housed withinthe enclosure.

FIG. 2A shows application of the gas sensor enclosure for gasmeasurement. In this embodiment, an unmodified gas is input directlyinto the sensor enclosure towards a sensor array through a one-wayinlet. Once within the enclosure, the gas is measured in the sensorarray and upon completion of the gas sensing, the sample exits theenclosure through a one-way exhaust outlet.

FIG. 2B shows application of the gas sensor enclosure for gasmeasurement followed by a flush mode. In this embodiment, an unmodifiedgas is input directly into the in-line filters to remove deleterious gasspecies and any remaining static and/or unfiltered gas analyte in theenclosure. Once sensing has completed and the measurement mode isfinished, the gas sample may be continually fed into the enclosure aspart of a “flush mode” where no sensing is taking place. The flush modeis designed to purge the enclosure of any contaminants or analyteremaining on the sensor array or in the enclosure that may acceleratesensor degradation, especially during idle periods, which occur betweensensing periods. During the flush mode, the enclosure routes theincoming sample (containing carrier gases, analytes, contaminants, etc.)through a series of filters in the enclosure to remove contaminants andother species that may degrade the sensors within the array. The removalof species present in the gas sample results in the flushing of theenclosure, including the gas sensor chamber, with contaminant-freecarrier gas resulting in the protection of the sensor array fromcontaminants, thus mitigating any degradation and loss of sensitivity tothe sensor array within the enclosure. The filters, as part of theenclosure, may be removed and replaced with new filters after a setperiod of time or once consumed or saturated. This embodiment allows forthe ongoing use of the sensing components and the enclosure, thusprolonging the use of a gas sensor in an environment where it wouldnormally degrade in a relatively short period of time.

FIG. 2C shows another application of the gas sensor enclosure for gasmeasurement followed by an “idle mode.” In this embodiment, the gas isinput into the device for measurement as provided in FIG. 2A, followedby the flush mode of FIG. 2B. During the idle mode, the input and outputvalves of the device are closed, effectively sealing off the sensorarray from inflowing mixtures that may compromise the sensor array.Because the closing off of the enclosure occurs immediately after theflush mode, the gases that have passed through the filters blanket thesensor, resulting in a significantly reduced chance of exposure of thesensor to materials that could degrade the sensitivity of the sensor orreduce its lifetime. Gas remaining in the idle mode is released from theenclosure by opening an exit valve that does not compromise the sensorarray (e.g., the exhaust near the input in FIGS. 2B and 2C).

With reference to FIG. 3 , in a further embodiment, a gas sensor housedwithin an enclosure has one input line with one or more filters that canmove as appropriate for the measurement, flushing, and idle modes of thegas sensor enclosure. In this embodiment, a sensor array is housedwithin an enclosure comprising a single input and a single output(exhaust) and only a single path for the gas to travel through. Similarto the embodiments of FIGS. 2A-2C, the measurement mode results inunencumbered flow of the gas through the sensor array and the subsequentexhaust of the sensed gas through the outlet. During the flush mode andidle modes, a retractable filter, which is retracted during themeasurement mode, is placed in line with the input so that the path ofincoming gas must flow through the filter before coming in contact withthe sensor array. Once sufficient flow through the enclosure has beenreached, or sufficient time has passed, the enclosure enters the idlemode where the input and the exhaust are closed off, sealing the sensorarray.

The design of the enclosure allows for the measurement and flushingmodes to occur quickly within a time period of one minute to no morethan 12 hours. The idle mode may remain in place until such time that anew gas is introduced into the enclosure for measurement by the gassensor.

One type of gas sensor in frequent use is a metal nanoparticle sensor.The sensing ability of metal nanoparticle sensors is dictated by theinter-particle distance, which is itself influenced by the choice offunctionalizing ligand. One ligand/metal nanoparticle combination thatis frequently used is organothiol ligands with gold nanoparticles, thelatter of which have the characteristics of strong S—Au bonds andchemical resistance. Like other chemiresistors currently in use,self-assembled monolayers of organothiols on flat gold surfaces degradein ambient conditions. Such degradation has also been observed onorganothiols covalently attached to gold nanoparticles, the latter ofwhich is used in Examples 1 and 2 to demonstrate that the protectiveenclosures described herein are capable of mitigating such degradation.

FIGS. 4-9 show the results of the experiments conducted in Examples 1and 2 on gold nanoparticles (Au-NPs) ligated to decanethiol orhexanethiol. FIGS. 4-7 are directed to Example 1, which tested theeffect that confinement has on the degradation of ligands surroundingAu-NPs; and FIGS. 8 and 9 are directed to Example 2, which tested theeffect of removing deleterious gases on a sensor enclosure. It is to beunderstood that the chemiresistive ligated gold nanoparticles were usedsolely as an example of a chemiresistor and are not meant to be limitingwith respect to any particular chemiresistor, gas sensor, and/or gassensor array that may be used with the enclosure described herein. Thegas sensor enclosure is designed to be used with any type of sensingtechnology where environmental exposure may decrease either the workinglifetime of a sensor and/or the sensitivity of the sensing technology.

FIG. 4A shows SERS (surface enhanced Raman spectroscopy) spectra ofdecanethiol-ligated Au-NPs directly after deposition, after 5 days ofstorage in a confined 25 mL volume of ambient air, and after 5 days ofstorage in freely exchanging ambient air. FIG. 4B shows SERS spectra ofhexanethiol-ligated Au-NPs directly after deposition, after 7 days ofstorage in a confined 25 mL volume of ambient air, and after 7 days ofstorage in freely exchanging ambient air. The decanethiol andhexanethiol ligands were covalently attached to the Au-NPs and providedcolloidal stability of the Au-NPs. As can be seen for both types ofligands (referred to collectively as “thiol ligands”), the Au-NPs storedin the confined volume (also referred to herein as “enclosed” or“sealed” conditions) showed a very similar SERS spectra to that offreshly deposited ligated gold nanoparticles (also referred to herein as“pristine” conditions), suggesting no significant ligand degradationtook place. By contrast, the particles stored in freely exchangingambient air (also referred to herein as “open-air” or “open ambient air”conditions) showed a completely different spectrum that is attributed tothe degraded thiol ligands. With reference to FIG. 4A, the peaks between1298-1123 cm⁻¹ and 1062-873 cm⁻¹ are generally ascribed to SOS-species,and the broad set of peaks ranging from 1750 cm⁻¹ to 1500 cm⁻¹, thestrong peak at 1378 cm⁻¹, and the sharp peak at 1000 cm⁻¹ are knownfingerprints for graphitic carbon, indicating that the decanethiolligand has degraded into SOS-species and graphitic carbon.

FIG. 5 shows XPS (x-ray photoelectron spectroscopy) spectra of the S2pand Si2s region of a layer of gold, pristine decanethiol-ligated Au-NPsstamped on a gold surface, and 40-day aged open-air decanethiol-ligatedAu-NPs stamped on a gold surface. Comparison of the pristine and agedpeaks shows that after the decanethiol-ligated Au-NPs were aged in openambient air, the Au—S peak of the pristine sample was replaced by an O—Speak, confirming the degradation of the decanethiol-ligated Au-NPs in anopen ambient air environment.

FIGS. 6A and 6B show SEM (scanning electron microscopy) micrographs ofhexanethiol-ligated Au-NPs in a pristine state immediately afterdeposition (FIG. 6A) and aged in open ambient air for 13 days (FIG. 6B).The aggregation of the individual nanoparticles confirms that Au-NPslose their protective ligand shells and agglomerate after exposure tofreely exchanging ambient air.

FIG. 7 shows the relative change in current (Response) over time ofsealed and aged decanethiol-ligated Au-NPs subjected for 400 seconds(t=100 sec to t=500 sec) to 12678 ppm of octane in an N₂ flow. Prior toexposure to the N₂ flow, the Au-NPs were stamped on interdigitatedelectrodes and stored in 25 mL of sealed ambient air for 14 days. Whilehighly conductive pristine Au-NPs lose their ability to sense afterexposure to open ambient air (not shown), sealed Au-NPs retain theirability to sense over prolonged periods of time, as is shown by thedistinct response of the Au-NPs to the introduction and withdrawal ofthe 12678 ppm of octane between t=100 and t=500 in FIG. 7 .

The results of Example 1 and FIGS. 4-7 demonstrate that storing sensorswithin an enclosed volume can extend the lifetime of the sensor. Example2 and FIGS. 8 and 9 address the effect that potential gaseous specieswithin the headspace of the enclosed volume may have on a sensor andmethods to eliminate deleterious gases from the enclosure that may leadto degradation of the sensor from within.

FIG. 8 shows SERS spectra of decanethiol-ligated Au-NPs in a pristinecondition immediately following deposition, after 24 hours of storage inan open-air environment, and after 24 hours of flushing with “zero-air”(a mixture of 80% nitrogen and 20% oxygen with no other species). Thezero-air sample, like the pristine sample, had no peaks attributable todegradation of the ligands. By contrast, the open-air sample displayed adegraded spectrum similar to the 5 and 7-day aged open-air samples ofFIGS. 4A and 4B, respectively. The results of FIG. 8 lead to theconclusion that open ambient air has one or more species that leads toligand degradation and that such one or more species are not present inzero-air.

FIG. 9A is a schematic depicting the set-up used in Example 2 tointroduce ozone contaminant into zero-air atmosphere and the transfer ofthe ozone-enriched zero-air to an Au-NP sensor. FIG. 9B shows SERSspectra of decanethiol-ligated Au-NPs after 24 hours of flushing withzero-air and after exposure to one hour of ozone enriched zero-air. Thespectra of the ozone-enriched decanethiol-ligated Au-NPs is similar tothe spectra of the decanethiol-ligated Au-NPs that were exposed to 24hours of open-air in FIG. 8 . The results of FIG. 9B confirm thedeleterious effect of ozone on chemiresistive ligated gas sensors. Inone application, a gas sensor enclosure may include one or more activecarbon filter to remove ozone contaminants.

The gas enclosure described herein allow for the effectiveimplementation of gas sensors that would normally suffer from a decreasein sensitivity or working life due to degradation resulting fromexposure to contaminants. Due to simplicity of design, the enclosed gassensing device does not require vacuum pumps or impractical powersupplies and can be manufactured as a small, lightweight, and low-powerportable device. The gas sensor enclosures have application for manyindustries, including, without limitation, incorporation into electronicnose and IoT devices that require accurate gas sensor that will provideaccurate gas sensing results and will not degrade over time.

The descriptions of the various aspects and/or embodiments of thepresent invention have been presented for purposes of illustration, butare not intended to be exhaustive or limited to the embodimentsdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art without departing from the scope and spiritof the described embodiments. The terminology used herein was chosen tobest explain the principles of the aspects and/or embodiments, thepractical application or technical improvement over technologies foundin the marketplace, or to enable others of ordinary skill in the art tounderstand the aspects and/or embodiments disclosed herein.

EXPERIMENTAL

The following examples are set forth to provide those of ordinary skillin the art with a complete disclosure of how to make and use the aspectsand embodiments of the invention as set forth herein. While efforts havebeen made to ensure accuracy with respect to variables such as amounts,temperature, etc., experimental error and deviations should be takeninto account. Unless indicated otherwise, parts are parts by weight,temperature is degrees centigrade, and pressure is at or nearatmospheric. All components were obtained commercially unless otherwiseindicated.

Example 1 Confinement Effect on Chemiresistive Ligated Gold NanoparticleSensors

To test the extent of degradation of chemiresistive ligated Au-NPs, 30nm Au-NPs were functionalized with decanethiol or hexanethiol ligandsand aged from 4 to 12 days in freely exchanging open-air and separatelyin a sealed 25 mL vial under air. Decanethiol-ligated andhexanethiol-ligated Au-NPs were also freshly prepared as pristinecontrols. The pristine, open-air, and sealed ligated Au-NPs were stampedon top of a gold surface for material characterization via SERS, XPS,and SEM and on top of interdigitated gold electrodes for resistivity/gassensing measurements.

Material Characterization Tests:

SERS was used to test the degradation of the ligands surrounding theAu-NPs. Open-air and sealed decanethiol-ligated Au-NPs aged for fivedays and open-air and sealed hexanethiol-ligated Au-NPs aged for sevendays were used for the tests (all ligated Au-NPs were stamped on a goldsurface). The SERS spectra results for the pristine, open-air, andsealed decanethiol-ligated Au-NPs are shown in FIG. 4A, and the SERSspectra results for the pristine, ambient, and sealedhexanethiol-ligated Au-NPs are shown in FIG. 4B. The SERS spectraresults showed significant degradation of the open-air Au-NP samples,whereas the sealed Au-NP samples had SERS spectra profiles that weresimilar to the profiles for the pristine samples. The SERS profiles weresimilar for the decanethiol-ligated Au-NPs and the hexanethiol-ligatedAu-NPs.

XPS was used to test the degree of degradation of the ligandssurrounding the Au-NPs. Pristine and open-air decanethiol-ligated Au-NPsstamped on the gold surface were used for comparative testing with a 100nm gold film used as a control. The open-air decanethiol-ligated Au-NPswere aged for 140+ days. The XPS results for the pristine and open-airAu-NP samples and the Au control are shown in FIG. 5 . The presence ofthe O—S peak in the open-air Au-NP sample, rather than the Au—S peakseen in the pristine Au-NP sample, indicated that the protectivedecanethiol ligands surrounding the Au-NPs had degraded during the 40+days of open ambient air storage.

SEM images were used to observe the effect that the degradation of theligands has on the nanoparticles. Hexanethiol-ligated Au-NPs were imagedimmediately after deposition on a gold surface and after aging for 13days in open ambient air. As shown in FIGS. 6A and 6B, the aged Au-NPsamples degraded and agglomerated as a result of the hexanethiol-ligatedAu-NPs losing their protective ligand shells after exposure to openambient air.

Resistivity Tests:

To test the charge transport resistant change that occurs during thedegradation of ligated Au-NPs, the resistance of hexanethiol-ligatedAu-NPs stamped on interdigitated gold electrodes was measured afterstorage for 4 days and 12 days in open ambient air and after the sameamount of time in 25 mL sealed containers. The resistance of pristinesamples was also measured. The results of the resistance change of thepristine, open-air, and sealed samples are shown in Table 1, along withthe resistance ratio of the pristine samples to the aged samples.

TABLE 1 Hexanethiol-ligated Au-NPs (Resistance Change in kΩ) Ratio:Ratio: Exposure 4 days 12 days Pristine/4 Pristine/12 ConditionsPristine aged aged days days Ambient Air 16000 kΩ  350 kΩ  0.12 kΩ 46133,333 Sealed Air  6200 kΩ 2930 kΩ  2170 kΩ 2.1 2.9 (25 mL)

The data in Table 1 show that the aggregation (i.e., ligand degradation)of hexanethiol-ligated Au-NPs is accompanied by drops in resistance froma pristine state to a 4-day old state (16000 kΩ to 350 kΩ) with afurther drop in resistance from day 4 to day 12 (350 kΩ to 0.12 kΩ). Thedrop in the resistance from 16000 kΩ to 0.12 kΩ between the pristinestate and the day 12 aging in open-air indicates that during the 12-dayperiod, the Au-NP layers became highly conductive as a result of thedegradation of the protective hexanethiol ligands surrounding theAu-NPs. By contrast, during the same 12-day period, the resistance ofthe Au-NP samples stored in the sealed containers changed from 6200 kΩto 2170 kΩ, indicating a much lower extent of ligand degradation.

Decanethiol-ligated Au-NPs stamped on interdigitated electrodes andsealed in a 25 mL volume container for 14 days were also tested fordegradation by introducing 12678 ppm of octane in an N₂ flow into thesealed container. The results of the degradation test are shown in FIG.7 , which plots current (Response) against time with the N₂ introductionoccurring between t=100 sec and t=700 sec. The results in FIG. 7 showthat the decanethiol-ligated Au-NPs reacted to both the influx and theremoval of the gas confirming that the decanethiol-ligated Au-NPs hadnot degraded during the 14-day aging process within the sealedcontainer.

Example 2 The Effect of Excluding Deleterious Compounds onChemiresisitve Ligated Gold Nanoparticle Sensors

To test for the presence of deleterious gases in a sensor environment,the following decanethiol-ligated Au-NPs stamped on top of a goldsurface were tested via SERS: a pristine sample; an open-air sample agedfor 24 hours; and a third sample exposed to a continuous stream of“zero-air” (i.e., a synthetic mixture containing 80% nitrogen, 20%oxygen, and no other species) for 24 hours. The SERS results shown inFIG. 8 confirm that the zero-air sample has a similar profile to thepristine sample indicating that the zero-air sample, like the pristinesample, had no ligand degradation. By contrast, the ambient air samplehad a SERS spectrum indicating degradation of the protective decanethiolligand by a substance in the open-air environment.

Since it is known that ozone can be responsible for Au—S bonddegradation, ozone was added to the zero-air mixture by introducing anozone generator between the zero-air source and thedecanethiol-litigated Au-NP sample, the latter of which had remainedunder a continuous stream of zero-air for 24 hours. A schematic of theozone zero-air set up is shown in FIG. 9A. The ozone generator wasswitched on and allowed to run for approximately 1 hour in order to formozone in the zero-air stream by UV illumination (the gold surfacesubstrate was not subjected to the UV light emitted by the ozonegenerator). A SERS spectrum of the ozone-exposed Au-NP sample wasrecorded and compared with the first 24-hour zero-air SERS spectrum thatshowed no ligand degradation. The SERS results shown in FIG. 9Bconfirmed that the introduction of the ozone into the zero-air streamresulted in a SERS spectrum that matched the open-air SERS spectrum ofFIG. 8 , both of which documented profiles indicating degradation of theligands surrounding the Au-NPs.

We claim:
 1. A device comprising: an enclosure; an inlet for input of agas; a sensing gas line with no filters; a purge gas line with at leastone filter to remove deleterious compounds from the gas; an inlet valveto open and close the inlet and direct flow of the gas into the sensinggas line and/or the purge gas line; a gas sensor with access to thesensing gas line and the purge gas line, wherein the gas sensor has (i)a first valve to cut off access of the gas sensor to the sensing gasline and (ii) a second valve to cut off access of the gas sensor to thepurge gas line; at least one outlet to exhaust the gas from the sensinggas line and the purge gas line; and at least one outlet valve to directthe exhaust from the sensing gas line and purge gas line out of theenclosure, wherein the at least one outlet valve is a one-way valve thatprevents outside air from entering the enclosure.
 2. The device of claim1, wherein the gas is an analyte of interest that enters the enclosurethrough the inlet, the inlet valve directs the analyte of interest intothe sensing gas line for measurement by the gas sensor, and aftermeasurement, the analyte of interest leaves the enclosure through one ofthe at least one outlet.
 3. The device of claim 1, wherein the gas is aflushing gas that enters the enclosure through the inlet, the inletvalve directs the flushing gas into the purge gas line, wherein theflushing gas passes through the at least one filter, over the gassensor, and exits the enclosure through one of the at least one outlet.4. The device of claim 3, wherein the inlet valve closes the inlet, andthe at least one outlet valve closes the at least one outlet, therebysealing the enclosure so that the flushing gas circulates within thesealed enclosure.
 5. The device of claim 1, wherein the gas sensor isselected from the group consisting of a metal oxide thin film, a metaloxide nanostructure, and a metal nanoparticle.
 6. The device of claim 1,wherein the gas sensor is a metal nanoparticle sensor, wherein the metalis selected from the group consisting of gold, silver, platinum, andpalladium.
 7. The device of claim 1, wherein the gas sensor is a metalnanoparticle ligated with a thiol.
 8. The device of claim 7, wherein thethiol is selected from the group consisting of ethanethiol, hexanethiol,octanethiol, decanethiol, dodecanethiol, and combinations thereof. 9.The device of claim 1, wherein the at least one filter is selected fromthe group consisting of activated carbon, silica gel, and combinationsthereof.
 10. The device of claim 1, wherein the at least one filter is areplaceable filter.
 11. A device comprising: an enclosure; an inlet forinput of a gas; an inlet valve for closing the inlet; at least oneretractable filter for removing deleterious compounds from the gas,wherein the at least one retractable filter is situated at the inlet; anoutlet for exit of the gas; an outlet valve for closing the outlet; anda gas sensor with access to the inlet and the outlet, wherein the gassensor is (i) sealed from the inlet via closure of the inlet valve and(ii) sealed from the outlet via closure of the outlet valve.
 13. Thedevice of claim 11, wherein the gas is an analyte of interest thatenters the enclosure through the inlet where the at least one filterremoves deleterious compounds from the gas, wherein the filtered gaspasses onto the gas sensor for measurement and, after measurement, theanalyte of interest leaves the enclosure through the outlet.
 14. Thedevice of claim 11, wherein the gas is a flushing gas that enters theenclosure through the inlet, passes over the gas sensor, and remainswithin the enclosure until released by the outlet, wherein upon entry ofthe flushing gas into the gas sensor, the inlet valve and the outletvalve close and seal the enclosure, thereby preventing escape of theflushing gas.
 15. The device of claim 11, wherein the gas sensor isselected from the group consisting of a metal oxide thin film, a metaloxide nanostructure, and a metal nanoparticle.
 16. The device of claim11, wherein the gas sensor is a metal nanoparticle sensor, wherein themetal is selected from the group consisting of gold, silver, platinum,and palladium.
 17. The device of claim 1, wherein the gas sensor is ametal nanoparticle ligated with a thiol.
 18. The device of claim 17,wherein the thiol is selected from the group consisting of ethanethiol,hexanethiol, octanethiol, decanethiol, dodecanethiol, and combinationsthereof.
 19. The device of claim 11, wherein the at least oneretractable filter is selected from the group consisting of activatedcarbon, silica gel, and combinations thereof.
 20. The device of claim11, wherein the at least one retractable filter is a replaceable filter.